Referring to FIGS. 20 and 21, structure and function of a conventional fluorescent display device having a getter will be described. FIGS. 20 and 21 show cross sectional views of the conventional fluorescent display device, respectively. In FIGS. 20A and 20B, reference numerals 70, 71 and 72 represent a side substrate, a first substrate and a second substrate, respectively, which constitute a vacuum vessel of a fluorescent display device.
Referring to FIG. 20A, a supporting member 74 is fixed to an insulation layer 73 formed on the second substrate 72, wherein a metal vessel 75 having getter material 76 filled therein is attached to the supporting member 74. When a laser beam is irradiated from outside of the second substrate 72 onto a bottom of the metal vessel 75, the getter material 76 is evaporated to thereby form a getter film on the first substrate 71 (e.g., see, Japanese Patent Laid-Open Publication No. 11-260262).
In this case, the metal vessel 75 is usually made of a nickel-plated steel vessel of a ring shape. The getter material 76, e.g., made of Ba, Al or Ni, is filled into the vessel 75. This type of getter is usually called as a ring-shaped getter.
Referring to FIG. 20B, a vessel 77 accommodating ball-shaped getter material 78 therein is installed at an opening part of the first substrate 71. When a selective heating is applied on the getter material 78 by employing a selective heating unit, e.g., a laser unit, the getter material 78 is activated (e.g., see, Japanese Patent Laid-Open Publication No. 10-64457).
In FIGS. 21A and 21B, reference numerals 80, 81 and 82 represent a side substrate, a first substrate and a second substrate, respectively, which constitute a vacuum vessel of a fluorescent display device. Referring to FIG. 21A, powder type getter material 83 is filled in a recessed portion formed in the second substrate 82. When a laser beam is irradiated on the getter material 83 from outside of the second substrate 82, the getter material 83 is evaporated to thereby form a getter film on the first substrate 81 (e.g., see, Japanese Patent Laid-Open Publication No. 5-114373).
Referring to FIG. 21B, a getter material layer 84 is formed on the second substrate 82 by employing, e.g., deposition technique. When a laser beam is irradiated on the getter material layer 84 from outside of the second substrate 82, the getter material layer 84 is evaporated to thereby form a getter film on the first substrate 81 (e.g., see, Japanese Patent Laid-Open Publication No. 5-114373). Besides a deposition technique, a paste coating technique for coating a paste mixed with getter material may also be employed to form the getter material layer 84 (e.g., see, Japanese Patent Laid-Open Publication No. 2-177234).
In a conventional fluorescent display device as shown in FIGS. 20A and 20B, there is needed a vessel accommodating getter material or a supporting member for supporting the vessel. An opening to attach the vessel to a substrate is also needed. Accordingly, fabrication cost of the vessel increases. The attachment of the vessel to the substrate is not technically easy. There is a limitation in an accommodating place of the vessel, e.g., the substrate in FIG. 20B. Further, since a considerable space is necessary for attachment of the vessel, a dead space increases, the dead space being a space which is not useful in a display function thereof.
In FIG. 20A, the vessel 75 of a particular shape and the supporting member 74 to attach the getter should be installed between the substrates 71 and 72. As a result, the size thereof becomes large and the structure thereof becomes complex; and the handling and attachment thereof become difficult. Especially, the handling or attachment of a thin fluorescent display device, e.g., having a space between two facing substrates smaller than 1.4 mm becomes difficult. Even if the attachment thereof is possible, since a distance between the getter material 76 and the first substrate 71 is small, evaporated getter material does not diffuse far away. Therefore, a getter film formed on the substrate 71 has small area and it is impossible to obtain full getter effect.
Since the fabrication cost of the vessel of a particular shape is high and the handling burden thereof is considerable, the manufacturing cost of the fluorescent display device becomes expensive. Further, spaces for the vessel of a ring shape and a getter attachment member become large, thereby entailing limitations in providing slim and small fluorescent display device.
In FIG. 20B, the thermal expansion coefficient of the first substrate 71 should be set as about equal to that of the vessel 77; and the first substrate 71 and the vessel 77 should be attached closely to each other to prevent the vacuum level of a vacuum vessel of the fluorescent display device being lowered. Therefore, it is necessary to fabricate the vessel 77 and an opening of the substrate attaching the vessel 77 in a high accuracy.
Referring to FIG. 21A, since a recessed portion should be formed in the second substrate 82, the substrate fabrication cost becomes high. Further, since the getter material to be filled in the recessed portion is powder, handling thereof is not easy and the filling procedure thereof is burdensome. Since the forming place of the recessed portion is limited within the substrate and a thin glass substrate of about 1 mm thickness is used in a thin fluorescent display device, the depth of the recessed portion is limited in view of the fact that a vacuum vessel thereof should be strong enough to endure an atmospheric pressure applied thereto. Accordingly, it is difficult to fill the recessed portion with the getter material in an amount required to form the getter film.
In the conventional fluorescent display device as shown in FIG. 21B, an expensive deposition unit is necessary for forming the getter material layer 84; and in forming the getter material, the patterning thereof is difficult. Further, since it is difficult to form the getter material layer 84 by employing a deposition technique on a component other than the substrate, the formation place of the getter material layer 84 is limited in the substrate.
Since the getter material layer 84 formed by employing a deposition technique is thin, the glass substrate may be locally over-heated depending on a radiation time duration of a laser beam irradiated thereon, thereby entailing a development of a crack in the substrate; and it is difficult to form the getter material layer in an amount required to form a getter film.
In the conventional fluorescent display device as shown in FIG. 21B, a paste coating technique instead of the deposition technique may be employed. However, when employing the paste coating technique, an expensive paste coating unit is needed; the pattering procedure in forming the getter material layer is difficult; and it is difficult to form the getter material layer on a component other than the substrate. Further, in this case, a mixture other than the getter material in the paste may be evaporated to thereby produce unnecessary gas.
For example, in manufacturing a fluorescent display device, e.g., made of an acryl, even if the paste is formed by employing a solvent such as one which is thermally decomposed in sealing and exhaust procedures, the adhesion force of the getter material is not sufficient. Accordingly, the getter material may be detached due to evaporation of the getter material or the vibration thereof.